Electrical spark producing apparatus



Nov. 16, 1965 w. w. SCHROEDER ETAL 3,218,498

ELECTRICAL SPARK PRODUCING APPARATUS Filed May 14, 1962 INVENTORS) dz IWonk Mayo United States Patent 3,218,498 ELECTRICAL SPARK PRODUCINGAPPARATUS Wolfgang W. Schroeder, Witfontein, Pretoria, Transvaal, andAlhertus Strasheim, Brooklyn, Pretoria, Transvaal, Republic of SouthAfrica, assignors to South African Council for Scientific and IndustrialResearch, Pretoria, Republic of South Africa Filed May 14, 1962, Ser.No. 194,240 Claims priority, application Great Britain, May 23, 1961,18,593/ 61 8 Claims. (Cl. 313-146) This invention relates to apparatusfor producing an electrical spark in a spark gap between two opposedaxially aligned electrodes.

In such apparatus it is often important to ensure that the commencementof the spark can be accurately timed.

It is an object of this invention to provide means which assist in suchaccurate timing of the commencement of the spark.

According to this invention there is provided apparatus for producing anelectrical spark in a spark gap between two opposed axially alignedelectrodes in which at least six needle-shaped members of electricallyconducting material are arranged so that their points are directedtowards the spark gap between the electrodes and so that their pointslie at equal intervals on the circumference of a circle around thecommon axis of the electrodes in a plane at right angles to such axisthe diameter and position of the circle being such that the said pointsare in close proximity to the spark gap but sufliciently spacedtherefrom so as to be free of the spark, electrical conmotions forapplying a sparking voltage to the electrodes, and electricalconnections adapted to apply to such needle-shaped members a voltage notless than that applied to the electrode operating at the higher voltage.

The needle-shaped members may conveniently be arranged around theelectrode operating at the lower voltage with the axes of their needlepoints at an acute angle to the axis of such electrode. Furthermore anelectrical conductor may be provided connecting the needle-shapedmembers to each other and to the electrode operating at the highervoltage.

In the apparatus according to this invention, there may be providedneedle adjustable mounting means for adjustably mounting theneedle-shaped members so that the diameter of the circle on thecircumference of which their points lie can be varied. Axial adjustmentmeans may be provided for supporting the needle adjustable mountingmeans for varying the position relative to the electrodes of the circleon the circumference of which the needle points lie. Electrode spacingadjustment means may be provided whereby the axial spacing between theelectrodes can be varied to vary the spark gap.

Gas flow means may also be provided for directing a flow of gas at thespark gap for removing therefrom electrically charged particlesresulting from one spark before the commencement of the next spark.

It is preferred to use eight needle-shaped members in the apparatus ofthis invention.

The apparatus of this invention assists in reducing the statistical timelag in the spark gap thereby assisting in the accurate timing of thecommencement of the spark.

The other well known factors which also assist in controlling thestatistical time lag in the spark gap must of course be suitablyadjusted in the conventional manner. Such other factors include thevoltage across the spark gap, the width of the spark gap, the electricalfield intensity in the spark gap as controlled, e.g. by the shape of theelectrode, the electrode material and the gas condition of the sparkgap.

Thus it is well known that the higher the voltage across 3,Zl8,498Patented Nov. 16, 1965 the gap becomes the shorter will be thestatistical time lag for a given gap width. The width of the spark gapis also known to have an influence such that the smaller the gap for agiven voltage the smaller will be the statistical time lag. Similarly itis well known that the shape of the electrodes affects the electricalfield intensity in the spark gap which in turn affects the statisticaltime lag. Generally, cylindrical electrodes with dome-shaped ends arepreferred. The material from which the electrodes are made should alsobe chosen appropriately in known manner. Thus materials with a low workfunction but not too high a rate of evaporation have been foundsuitable. Aluminum is the preferred material. The gas condition in thespark gap is also known to affect the statistical time lag. Thus foraccurately timed repetitive sparks it is known to be desirable to ensureas far as possible that there will be identical gas conditions obtainingin the spark gap just prior to the commencement of each spark. It istherefore preferred to blow electrically charged particles produced byone spark out of the spark gap before the commencement of the nextspark, by means of a suitable blast of gas.

The needle-shaped members provided in the apparatus of the presentinvention are believed to assist in the reduction of the statisticaltime lag by providing a corona discharge effect from their needle pointsproducing sufficient ultra-violet radiation to cause the appearance ofhigh energy electrons in the spark gap. So also for repetitive sparksthe needle-shaped members are believed to mask changes in the electricalfield conditions in the spark gap arising from haphazard cratering ofthe electrodes by the spark, thereby providing more nearly identicalelectrical field conditions for each spark.

The adjustable features of the preferred form of the apparatus accordingto this invention enable the setting of the needle-shaped members inoptimum position causing minimum spark jitter, for each of a number ofdilferent operating conditions, e.g. applied voltage, spark gap width,electrode materials, etc.

In view of the fact that in the apparatus of this invention the needlepoints are spaced sufficiently from the spark gap so as to be free ofthe spark, i.e. so that the spark does not strike the needle points, itis possible to use the apparatus for a considerable time beforereadjustments are required, and the needles have a comparatively longlife.

The apparatus of this invention finds a particularly useful applicationin time resolved spectrochemical analysis using a control gap with oneor more analytical gaps in any conventional electrical circuitarrangement. As is well known in such analysis the control spark gap isemployed to control as accurately as possible the time of commencementof the repetitive sparks in the analytical spark gap where the materialto be analyzed is rendered luminous by the sparks and viewed throughspectral analyzing equipment. It is therefore desirable that thecommencement of the spark in the control gap should be controlled asaccurately as possible and that the spark in the analytical gap shouldcommence as nearly as possible at the same time as the spark in thecontrol gap. This may to a large measure be achieved by using theapparatus of the present invention as the control gap and disposing theanalytical gap(s) in close proximity to such control gap Vgith anoptical path for ultra-violet radiation between t em.

As pointed out above the apparatus of the present invention enables thecommencement of the spark in the control gap to be controlled to a highdegree of accuracy and the ultra-violet radiation produced by the sparkin the control gap can be made to trigger the spark in the analyticalgap. As ultra-violet radiation travels with the speed of light and asthe analytical gap is disposed in J close proximity to the control gapit follows that the spark in the analytical gap will commence virtuallyat the same time as the spark in the control gap. The analytical gapmust not be so close to the control gap as to be contaminated by thespark products of the control gap. Furthermore the optical path betweenthe control gap and the analytical gap must be such as to passultra-violet radiation. Thus while a quartz window would be permissiblein such optical path a glass window would not, as ultraviolet radiationcannot pass therethrough.

An arrangement such as described above allows greater freedom ofadjustment in the analytical gap which facilitates the whole process ofanalysis. Thus a greater variation in electrode material or shape and ingap width will be permissible without affecting the accurately timedcommencement of the spark in the analytical gap.

By way of example the invention will now be described with reference tothe accompanying drawings in which:

FIGURE 1 is a cross-sectional elevation of one embodiment of thisinvention;

FIGURE 2 is a perspective view of a connecting ring; and

FIGURE 3 is a side elevation of a needle point adjustment former.

Referring to FIGURE 1, the spark gap is defined between the cylindricalaluminum electrode 1 operating at the higher voltage and cylindricalaluminum electrode 2 operating at the lower voltage. Electrode 1 ismounted in a brass holder 4 all supported by an insulating column 5 ofpolymethyl methacrylate (perspex) on an insulating base 18 of similarmaterial which is clamped to the side support of any suitable arc stand19. The electrical connection'to electrode 1 is made at terminal 6.Electrode 2 is mounted removably in metal control support 11 passingthrough base 18 and having at the other end thereof terminal 22 by whichthe electrical connection for electrode 2 is made. Mounted on controlsupport 11 are two adjustment discs 12 and 13.

Adjustment disc 12 is made of electrically insulating material and hasinternal screw threads cooperating with external screw threads oncontrol support 11. Mounted slidably on control support 11 is a metalsleeve 23 with a circular shouldered flange 24 at its lower end.Rotation of the metal sleeve 23 is prevented by the key in theshouldered flange thereof engaging slidably in the key way 11a providedtherefor in control support 11. Spring 14 bears on the top of theshouldered flange 24 of metal sleeve 23 and against the bottom shoulderprovided on head 25 of control support 11.

Mounted on the shouldered flange 24 of metal sleeve 23 is a perspex tube7 in the top end of which are provided eight steel needles 3 with theirpoints directed towards the spark gap between the electrodes 1 and 2.These steel needles are spaced equally around the perspex tube 7 andtheir axes are at an angle of 60 to the common axis of the electrodes 1and 2. The set screws 8 and the tube 7 form part of needle adjustablemounting means for adjusting the diameter of the circle defined by thepoints of the needles 3. The needles 3 can be set at variable positionsin their mountings by set screws 8 all passing through connecting ring 9made of steel. This connecting ring 9 is mounted on the top edge ofperspex tube 7 and serves as an electrical connection to all the needles3. Flexible conductor 26 connects connecting ring 9 to terminal 27 inbrass holder 4.

Adjustment disc 12, and tube 7 form part of axial adjustment means forvarying the axial position of the circle defined by the points of theneedles 3, relative to the electrodes 1 and 2. By rotating adjustmentdisc 12 in one direction metal sleeve 23 is raised so as to compressspring 14 and to raise perspex tube 7 and with it needles 3 relative toelectrode 2. By rotating adjustment disc 12 in the opposite directionspring 14 forces metal sleeve 23 down and with it perspex tube 7 andneedles 3 are lowered relative to electrode 2.

Adjustment disc 13 is made of electrically insulating material and hasinternal screw threads cooperating with external screw threads oncontrol support 11. Adjustment disc 13 abuts on the one side against theinside of metal bracket 28 mounted with screws against the bottom ofperspex base 18 and on the other side against metal sleeve 29 passingthrough perspex base 18 and slidably surrounding control support 11.Rotation of adjustment disc 13 in one direction raises control support11 and with it adjustment disc 12, metal sleeve 23, perspex tube 7 andelectrode 2 thereby decreasing the spark gap. Similarly rotation ofadjustment disc 13 in the opposite direction increases the spark gap.Adjustment disc 13, metal bracket 28, and metal sleeve 29 form part ofelectrode spacing adjustment means whereby the spacing betweenelectrodes 1 and 2 and hence the spark gap may be varied.

Metal arm 16 is non-rotatably mounted on control support 11 and has aforked end slidably engaging pin 17 fixed in base 18. This metal armprevents rotation of control support 11 when adjustment discs 12 and 13are rotated.

Perspex tube 20 is mounted on column 5 so as to direct an airstream(e.g. supplied at 3 to 5 lbs. pressure when tube 20 has a diameter of Ainch) at the spark gap. Clamping screw 21 clamps column 5 to base 18 andwhen released allows column 5 and brass holder 4 to be swung out to oneside to facilitate removal, replacement or adjustment of electrodes 1and 2 and tube 7 with needles.

The points of needles 3 are all set on the circumference of a circlearound the axis of the electrodes and in a plane at right angles to suchaxis. This can conveniently be achieved by means of a needle pointadjustment former 10. Electrode 2 is removed, former 10 inserted in itsplace and the needles 3 set by set screws 8 with their points in contactwith the cylindrical surface of the former. A set of formers withdifferent diameters can be provided to allow for a number of differentadjustments of the needle points.

When operating this apparatus as the control gap in conventionalspectrochemical analysis using one analytical gap, with 6.00 mm.diameter electrodes, a spark gap of 4.5 mm., an applied voltage of 17kilovolts across the spark gap and applying repetitive sparks of 50, upto say 400 sparks per second it has been found that the optimum diameterof the circle on which the needle points lie is 3 cm. and that thiscircle should be positioned 2 /2 cm. below the centre of the spark gap.A capacitance of about 100 picofarads is connected between the two leadsto the analytical gap to avoid small changes in the setting of theelectrodes causing a significant change in the capacitance leading tothe spark.

Under such conditions the spark jitter can be reduced to less than 10millimicro seconds, i.e. the commencement of each spark can be timedwith an error of less than 10 millimicro seconds. Where the analyticalgap is arranged, as described above, to be triggered by the ultra-violetradiation of the control gap (e.g. about 10 to 15 cm. from the controlgap), it is clear that greatly improved analytical conditions areobtained by virtue of the great accuracy with which the commencement ofthe spark in the analytical gap can be timed. Furthermore it was foundthat under such operating conditions the width of the analytical gapcould be varied from 3 to 12 millimetres with varied electrode materialsand shapes which greatly facilitates the analytical operations.

In known high precision high voltage electronical control sparkapparatus, a special type of high voltage transformer must be used. Thisinvention allows the use of standard conventional high voltagetransformers used for building spectro-chemical spark light sources.

For different operating conditions the optimum setting of the variousvariable adjustments of the apparatus described above can be readilydetermined.

Wherever in this specification and claims it is stated that the pointsof the needle-shaped members are directed towards the spark gap betweenthe electrodes it must be understood that it does not necessarily meanthat they must be directed to the centre of the spark gap but includesthe cases where they are directed slightly above or below the spark gap.

What we claim is:

1. Apparatus for producing an electrical spark in a spark gap betweentwo opposed elongated electrodes arranged with their longitudinal axesin alignment, in which at least six needle shaped members ofelectrically conducting material are arranged so that their points aredirected towards the spark gap between the electrodes and so that theirpoints lie at equal intervals on the circu'mference of a circle aroundthe common axis of the electrodes and in a plane at right angles to suchaxis the diameter and position of the circle being such that the saidpoints are in close proximity to the spark gap but sufiiicently spacedtherefrom so as to be free of the spark, electrical connections forapplying a sparking voltage to the electrodes, and electricalconnections adapted to apply to such needle shaped members a voltage atleast equal to that applied to the electrode operating at the highervoltage.

2. Apparatus according to claim 1 in which the needleshaped members arearranged around the electrode operating at the lower voltage with theirneedle points disposed in a circle around the external circumference ofthe electrode and with the axes of their needle points at an acute angleto the axis of such electrode.

3. Apparatus according to claim 1 which includes an electrical conductorconnecting the needle-shaped mem bers to each other and to the electrodeoperating at the higher voltage.

4. Apparatus according to claim 1 in which there is provided needleadjustable mounting means for adjustably mounting the needle-shapedmembers so that the diameter of the circle on the circumference of whichtheir points lie can be varied.

5. Apparatus according to claim 1 in which there is provided axialadjustment means supporting the needle adjustable mounting means forvarying the position relative to the electrodes of the circle on thecircumference of which the needle points lie.

6. Apparatus according to claim 5 in which there is provided electrodespacing adjustment means; whereby the axial spacing between theelectrodes can be varied to vary the spark gap.

7. Apparatus according to claim 1 in which gas flow means are providedfor directing a flow of gas at the spark gap for removing therefromelectrically charged particles resulting from one spark before thecommencement of the next spark.

8. Apparatus according to claim 1 arranged as the control gap with atleast one analytical gap for time resolved spectrochemical analysis inwhich the analytical gap is disposed in close proximity to the controlgap with an optical path for ultra-violet radiation between the controlgap and the analytical gap.

References Cited by the Examiner UNITED STATES PATENTS 2,703,374 3/ 1955Fruengel 313l46 X 3,085,174 4/ 1963 Van Tol et al 313-196 FOREIGNPATENTS 831,711 2/ 1952 Germany.

JOHN W. HUCKERT, Primary Examiner.

ARTHUR GAUSS, Examiner.

1. APPARATUS FOR PRODUCING AN ELECTRICAL SPARK IN A SPARK GAP BETWEENTWO OPPOSED ELONGATED ELECTRODES ARRANGED WITH THEIR LONGITUDINAL AXESIN ALIGNMENT, IN WHICH AT LEAST SIX NEEDLE SHAPED MEMBERS OFELECTRICALLY CONDUCTING MATERIAL ARE ARRANGED SO THAT THEIR POINTS AREDIRECTED TOWARDS THE SPARK GAP BETWEEN THE ELECTRODES AND SO THAT THEIRPOINTS LIE AT EQUAL INTERVALS ON THE CIRCUMFERENCE OF A CIRCLE AROUNDTHE COMMON AXIS OF THE ELECTRODES AND IN A PLANE AT RIGHT ANGLES TO SUCHAXIS THE DIAMETER AND POSITION OF THE CIRCLE BEING SUCH THAT THE SAIDPOINTS ARE IN CLOSE PROXIMITY TO THE SPARK GAP BUT SUFFICIENTLY SPACEDTHEREFROM SO AS TO BE FREE OF THE SPARK, ELECTRICAL CONNECTIONS FORAPPLYING A SPARKING VOLTAGE TO THE ELECTRODES, AND ELECTRICALCONNECTIONS ADAPTED TO APPLY TO SUCH NEEDLE SHAPED MEMBERS A VOLTAGE ATLEAST EQUAL TO THAT APPLIED TO THE ELECTRODE OPERATING AT THE HIGHERVOLTAGE.